Method and time-of-flight camera for providing distance information

ABSTRACT

The invention relates to a method for providing distance information of a scene with a time-of-flight camera ( 1 ), comprising the steps of emitting a modulated light pulse towards the scene, receiving reflections of the modulated light pulse from the scene, evaluating a time-of-flight information for the received reflections of the modulated light pulse, and deriving distance information from the time-of-flight information for the received reflections, whereby a spread spectrum signal is applied to a base frequency of the modulation of the light pulse, and the time-of-flight information is evaluated under consideration of the a spread spectrum signal applied to the base frequency of the modulation of the light pulse. The invention further relates to a time-of-flight camera ( 1 ) for providing distance information from a scene, whereby the time-of-flight camera ( 1 ) performs the above method.

The present invention relates to a method for providing distanceinformation of a scene with a range finding device such as atime-of-flight sensor or time-of-flight camera, comprising the steps ofemitting a modulated light pulse towards the scene, receivingreflections of the modulated light pulse from the scene, evaluating atime-of-flight information for the received reflections of the modulatedlight pulse, and deriving distance information from the time-of-flightinformation for the received reflections. The present invention alsorelates to a range finding device such as a time-of-flight sensor ortime-of-flight camera for providing distance information from a sceneaccording to the above method.

BACKGROUND

A time-of-flight camera, also referred to as TOF camera, is a camerathat usually comprises a light source or emitter unit for emittingmodulated light pulses, a receiver unit that captures reflections of thelight pulses, an evaluation unit, which evaluates time-of-flightinformation or the received reflections, and a calculation unit, whichderives distance information from the time-of-flight information. Thedistance information is also referred to as depth. The emitter unitemits a modulated light pulse towards a scene, whereby the modulatedlight pulse is reflected from objects present in the scene towards thereceiver unit. Depending on the distance of the objects from the TOFcamera, the reflections are received with a delay in respect to theemitted modulated light pulse. This delay, also referred to astime-of-flight, is evaluated by the evaluation unit and furtherprocessed by the calculation unit to derive a distance of the objects.

The receiver unit comprises several light receiving points, alsoreferred to as pixel, and an optical system, so that different pixelscan receive reflections from different objects at the same time. Each ofthe pixels independently receives reflections of the modulated lightpulses. Also the time-of-flight information and distance information areprocessed individually for each pixel, so that the distance informationis provided for the entire scene simultaneously

The distance is measured pixel per pixel in an indirect manner bymeasuring the time delay or phase difference between sent and receivedmodulated optical signal. Typically, the modulation can be a pulsedmodulation, sinusoidal modulation, pseudo-noise modulation, etc. Thephase difference of the sent and received modulation signal then offersa measure for the time delay.

To provide a TOF camera that can provide distance information with highaccuracy, sharply defined pulses are used, which have preferably alimited rise and fall time. Due to this requirement the spectral contentof the electronic modulation signal used to modulate the light output ofelectronic light sources contains a lot of harmonics with significantenergy, as shown in the example spectrum of FIG. 1.

Also, the modulation frequency is usually well defined since thetransformation from phase measurement to distance measurement requiresaccurate knowledge of the modulation frequency. Due to this, the peaksin the spectrum tend to be narrow but high.

Problems can arise, since TOF cameras must co-exist with otherelectronic devices. A lot of energy in the harmonics can preventelectromagnetic compliance of the device. Normally such a device has tocomply with different norms. Relevant norms for electromagneticcompliance (EMC) are available according to the FCC (USA), EC (Europe)or CCC (China) standards. Accordingly, the use of TOF cameras can berestricted and/or the accuracy of TOF cameras can be limited indirectlyby the requirement to fulfill EMC requirements. Typical modulationfrequencies are between 10 MHz and 100 MHz, generating relatedharmonics.

Reducing the amount of EMI radiated from an electronic device is one ofthe hardest problems to resolve in the drive for lower production costs.Producing a compliant device can be quite costly if the necessary stepsare not taken at design time. It is possible that 40-50% of thedevelopment cost of a new product can be spent in the quest for acompliant product suitable for economic production.

A clock oscillator generates a fixed frequency square wave signal usedfor timing in high-speed digital systems. The frequency of this clock isassumed to be fixed and taking the inverse of frequency gives the periodof the clock. The period of the clock is the time from a point on therising edge to the exact same point on the rising edge of the very nextclock. An ideal clock would have no measurable jitter and the period ofeach clock cycle would always be exactly the same. A Low EMI clockoscillator or Spread Spectrum Clock is a special type of digital clockthat provides lower EMI when compared with conventional clock generatoroutputs. The base frequency is modulated and the energy is spread outover a wider spectrum of frequencies, thereby reducing the peak energycontained at any one frequency. The peaks of the fundamental andharmonic frequencies are lower in relative strength. The total amount ofenergy that was originally in the harmonics of the base frequency clocksignal does not simply disappear but rather is distributed over a widerband of frequencies. By varying the frequency of a clock, the period ofsuch a clock is also changed which is the same as providing jitter. Thuscycle-to-cycle jitter is added to such a clock. Jitter will reduce theprecision of range finding time-of flight devices in determiningdistance.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide analternative method for providing distance information of a scene with arange finding device such as a time-of-flight sensor or a time-of-flightcamera, as well as an alternative range finding device such as atime-of-flight sensor or time-of-flight camera adapted to providedistance information from a scene according to the above method.

This object is achieved by the independent claims. Advantageousembodiments are detailed in the dependent claims. An advantage ofembodiments of a method and of a range finding device such as atime-of-flight sensor or a time-of-flight camera is that the distanceinformation is obtained with high accuracy while reducing EMI. Hence anadvantage of embodiments of a method and of a range finding device suchas a time-of-flight sensor or a time-of-flight camera according to thepresent invention is that compliance with EMC standards whilemaintaining high accuracy.

Accordingly, the above object is achieved by a method for providingdistance information of a scene with a range finding device such as atime-of-flight sensor or a time-of-flight camera, comprising the stepsof emitting an periodic light signal such as a train of light pulsestowards the scene, receiving reflections of the periodic light signalfrom the scene, evaluating a time-of-flight information for the receivedreflections of the periodic light signal, and deriving distanceinformation from the time-of-flight information for the receivedreflections, whereby the periodic light signal is emitted in accordancewith a modulation signal at a base frequency whereby a perturbation isapplied to a base frequency of the modulation signal as a frequencymodulation. This frequency perturbed modulation signal is then used togenerate the periodic light signal and is also used as a referencesignal by the detector receiving the reflected light for determining thetime-of-flight information. This can be done on a pixel-by-pixel basis.From this distance information as well as an image of the scene, a 3Dimage can be generated.

Accordingly in one aspect the present invention a method for providingdistance information of a scene is provided comprising the steps of:

-   -   emitting a periodic light signal towards the scene in accordance        with a clock timing that has a base frequency spread by a        periodic perturbation with a perturbation frequency and period,    -   receiving reflections of the periodic light signal from the        scene,    -   evaluating a time-of-flight information for the received        reflections of the periodic light signal over a set of a        plurality of measurement durations in accordance with the clock        timing that is spread by a periodic perturbation as a reference        signal, and    -   deriving distance information from the time-of-flight        information for the received reflections, wherein each        measurement duration of the set is an integer or half integer        multiple of the perturbation period and over the set of        measurement durations the average base frequency is kept        constant.

The periodic light signal can be pulses, such as square wave pulses butalso other waveforms such as sinusoidal signals.

The object is also achieved by range finding device such as atime-of-flight sensor or a time-of-flight camera for providing distanceinformation from a scene, whereby the time-of-flight camera performs theabove method.

Accordingly in another aspect of the present invention a time-of-flightsensor for use with a light source for emitting a periodic light signaltowards a scene is provided, the sensor being for providing distanceinformation from the scene, the sensor comprising:

a modulation unit for providing a modulation signal to the light sourcebased on a clock timing with a base frequency spread by a periodicperturbation having a perturbation frequency and period,a reception group with a receiver unit, an evaluation unit and aprocessing unit, the reception group being connected to the modulationunit to receive the modulation signal, the evaluation unit being adaptedto evaluate time-of-flight information from received reflections fromthe scene over a set of a plurality of measurement durations inaccordance with the clock timing that is spread by the periodicperturbation as a reference signal, the calculation unit being adaptedto derive distance information from the time-of-flight informationprovided by the evaluation unit wherein each measurement duration of theset is an integer or half integer multiple of the perturbation periodand over the set of measurement durations the average base frequency iskept constant.

The modulation signal can be pulses, such as square wave pulses but alsoother waveforms such as sinusoidal signals.

The present invention also provides a timing module for time-of-flightsensor for use with a light source for emitting a periodic light signaltowards a scene, the sensor being for providing distance informationfrom a scene, the timing module comprising:

a modulation unit (3) for providing a modulation signal for the lightsource based on a clock timing with a base frequency spread by aperiodic perturbation with a perturbation frequency and period,the modulation being adapted to provide the modulation signal over a setof a plurality of measurement durations in accordance with the clocktiming that is spread by the periodic perturbation,wherein each measurement duration of the set is an integer or halfinteger multiple of the perturbation period and over the set ofmeasurement durations the average base frequency is kept constant.

The modulation signal applied to the light sources can be pulses, suchas square wave pulses but also other waveforms such as sinusoidalsignals.

One aspect of the present invention is to apply a periodic frequencyperturbation to the base frequency of the modulation of the light and toevaluate the time-of-flight information for reflections underconsideration of this perturbation whereby each measurement duration ofa set of measurement durations needed to determine one phase differencevalue is an integer or half integer multiple of the perturbation period.In addition the average base frequency is kept constant over eachmeasurement duration of the total measurement durations needed todetermine one phase difference value despite the perturbation. Theperiodic perturbation excludes random perturbations of frequency whichwould not have the same spectral content even if they would provide thesame average frequency. The perturbation can be a continuous oscillatingvariation in base frequency or a sequence of different frequenciescentered around the base frequency.

Despite the perturbation of the base frequency of the periodic lightsignal introducing some effective jitter, the evaluation of thetime-of-flight information for reflections can be done with highaccuracy. Objects, which are present in the scene, can be reliablydetected and suitable distance information can be provided for theseobjects. The applied frequency perturbation improves the electromagneticcompliance of the range finding device such as a TOF sensor or the TOFcamera. It has the effect that some high-energy fundamental or harmonicpeaks are no longer located at sharply defined frequencies in thespectrum. The energy in the peaks is spread over a larger spectralregion, lowering the spectral energy density and thus improving the EMC.This technique results not only in a “spread spectrum” technique appliedto the periodic light signal but also maintains the spectral contentconstant over the set of measurement durations needed to determine onephase difference value. Spread spectrum clocks with periodic clocktiming perturbations, are readily available but these introduce somejitter. They can be incorporated into TOF sensors or TOF cameras withthe novel modifications of embodiments of the present invention.

The time-of-flight information of the received reflections is determinedby measuring the time delay or phase change between the receivedperiodic light signal and the reference signal, both having experiencedthe same perturbation. Due to this homodyne measurement principle, theperturbation potentially does not affect the time-of-flight measurement.The received reflections, which are correlated or mixed due to theoverlap of the emitted and the received pulses, can be easily evaluatedin respect to the time-of-flight information and the distanceinformation can easily be derived from this time-of-flight information.In one embodiment a TOF sensor or a TOF camera, preferably a spreadspectrum clock or a single system clock together with a singleperturbation clock, is used for generating the modulation signal forgeneration of the periodic light signal and also in the evaluation ofthe time-of-flight information. The perturbation is a periodic signal,e.g. it is a signal with a low frequency compared to the base frequencyof the modulation.

According to a preferred embodiment of the invention applying theperturbation to the base frequency of the modulation comprises modifyingthe base frequency of the modulation within an interval of +/−5% of thebase frequency of the modulation, preferably within an interval +/−1.5%of the base frequency of the modulation, even more preferably within aninterval +/−0.1% of the base frequency of the modulation. The size ofthe interval influences how strongly the energy in the harmonics isspread in the spectrum and accordingly influences the energy density ofthe spikes in the frequency spectrum. The bigger the interval, the lowerthe energy density of the harmonics. This results in the TOF sensor orthe TOF camera being able to be complaint with EMC regulations moreeasily.

In a preferred embodiment of the present invention the time-of-flightinformation of a scene is measured in a sequence of acquisitions. Thecombination of these different acquisitions allows removal of ambientlight influence and removal of object reflectivity and other sourcesdisturbing the time-of-flight measurement. The result of theacquisitions is a determination of the time delay or phase changebetween the emitted and received light. The steps for such acquisitionincludes: emitting a periodic light signal towards the scene andreceiving reflections of the periodic light signal from the sceneperformed sequentially in the given order at least twice, and the stepof evaluating a time-of-flight information for the received reflectionsof the periodic light signal which comprises integrating the receivedreflections of the periodic light signal over all performances of thestep of receiving reflections of the periodic light signal from thescene. By integrating the received reflections over a duration of thereflected periodic light signal, the time-of-flight can be evaluatedmore accurately. Reflections from objects far away can be easilydetected, even though the reflection of the periodic light signal fromthe object has only a low intensity. Hence, a “deep” scene with distantobjects as well as objects close by can be covered. Also, the integralevaluation results in an averaging of the received reflections over aduration which avoids isolated errors. Preferably, the integration isperformed over the reflections of at least three performances of thesteps of emitting the periodic light signal and receiving thereflections from the scene. Problems can occur when the spectral contentof the periodic light signal changes over the period of measurement forone time of flight distance determination. For this reason methods andapparatus of the present invention are adapted to keep the spectralcontent the same over the complete measurement period required for theacquisition of one distance determination.

Firstly, according to a preferred embodiment of the present inventionall acquisitions required for a time-of-flight measurement are obtainedusing the same average frequency. The average frequency preferably isthe frequency, around which the base frequency is changed due to theapplied perturbation. This requires a balanced or center spreadperturbation. The essentially identical average base frequency allows anaccurate processing of the received reflections of the emitted periodiclight signal into valid depth information.

In a preferred embodiment of the present invention each acquisitionrequired for a time-of-flight measurement experiences the same periodiclight signal, including the same perturbation of the base frequencysignal. The result is the same spectral content for each acquisition.The perturbation can be a known and repeated sequence of change of thebase frequency, and this known perturbation sequence is applied to thebase frequency in the same way for all acquisitions in the ToFmeasurement. Accordingly, as the frequency perturbation is periodic andthe measurement duration is an integer or half integer number ofperturbation periods, the emission of the periodic light signal isstarted always at the same position in the perturbation signal withinthe perturbation period or with a half period offset. As half periodplus an integer number of periods is the same as the integer number ofperiods plus a half a period, this also provides the same spectralcontent as a pure integer number.

There is no need for synchronisation between the period of the basefrequency and the perturbation period when the difference in frequencybetween the base and perturbation frequencies is high as the errorinduced is very small. Thus the same “spectral content” is notsynonymous with “exactly identical spectral content”. Any difference ispreferably smaller than the noise floor of the system. For example,differences of the order of less than 0.1% or 0.01% can be tolerated.Synchronisation between the base period and the perturbation period isnot excluded from the present invention when it is required foradditional accuracy.

Preferably, a continuous perturbation signal can be used. According toanother embodiment of the invention the perturbation is a discontinuousmodulation applied to the base frequency of the periodic light signal.It is merely required, that the perturbation modulation is aperiodically repeated signal and that the average frequency of theresulting perturbed signal is known. It is preferred that the averagefrequency is the center frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are illustrated in theaccompanied figures. These embodiments are merely exemplary, i.e. theyare not intended to limit the content and scope of the appended claims.

FIG. 1 shows an EMC measurement of a time-of-flight camera.

FIG. 2 shows a time diagram of a modulation signal and a lower frequencyperturbation signal according to an embodiment of the present invention,

FIG. 3 shows a time diagram of the modulation signal according to anembodiment of the present invention, and

FIG. 4 shows a schematic diagram of a time-of-flight camera according toan embodiment of the present invention.

FIG. 5 shows a schematic diagram of a timing module for a time-of-flightcamera according to an embodiment of the present invention.

FIG. 6 schematically shows an implementation of a TOF camera accordingto an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where an indefinite or definite article is usedwhen referring to a singular noun e.g. “a” or “an”, “the”, this includesa plural of that noun unless something else is specifically stated. Inthe different figures, the same reference signs refer to the same oranalogous elements. The illustrations in the figures are schematic.

The term “comprising”, used in the claims, should not be interpreted asbeing restricted to the means listed thereafter; it does not excludeother elements or steps. Thus, the scope of the expression “a devicecomprising means A and B” should not be limited to devices consistingonly of components A and B. It means that with respect to the presentinvention, the only relevant components of the device are A and B.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

The present invention will be described with reference to a TOF camerabut the present invention also includes the provision of any kind ofrange finding device working on the Time of Flight principle, a TOFsensor, e.g. with only one pixel, etc. Further the TOF camera or the TOFsensor is not necessarily delivered with an integral light source. Thelight source and its energy supply and drivers can be providedseparately and the camera or sensor only needs to comprise circuitry toprovide signals for modulating the light source.

An embodiment of the present invention will be described with referenceto FIG. 5 which is a schematic block diagram of a timing module 20 thatcan be used with a TOF camera. This embodiment has a clean system clock22, running at, for example, 80 MHz, e.g. in the range 10 to 320 MHz,and will be called the ‘clean clock’. The clock signal of the cleanclock 22 is sent to a Spread Spectrum block 24, which spreads thespectrum of this clock signal using a periodic spreading function,called the ‘perturbation frequency modulating signal’. Optionally, othercomponents can be placed between the clean clock 22 and the spreadspectrum block 24, such as filters, wave shapers, frequency converters,phase locked loops, etc. The output of this Spread Spectrum block iscalled the ‘spread clock signal’. Optionally, other components can beplaced after the spread spectrum block, such as filters and waveshapers, e.g. to generate a modulation signal with a desired waveformsuch as sinusoidal.

The perturbation frequency is within +/−5% of the base frequency of themodulation, preferably within an interval +/−5 or +/−1.5% of the basefrequency of the modulation, or within an interval +/−0.1% of the basefrequency of the modulation. The periodic perturbation can have asinusoidal or triangular waveform or saw tooth form for example.

The timing module 20 can be adapted to supply a discontinuous modulationto the light sources.

The spread clock signal is used by the Mixing signal and illuminationgeneration block 26, which generates the required TOF light source drivesignals at a typically, perhaps 2 or 4 times lower frequencies. TheseTOF signals include the signal used to modulate the light source, calledthe ‘light modulating signal’, as well as the mixing signals required bythe sensor in order to be able to de-modulate the incoming light,reflected by the scene. Thus the same TOF timing signals are sent toboth an illumination unit comprising light sources as well as thedetector used for detecting received reflected light. Optionally, othercomponents can be placed after the Mixing signal and illuminationgeneration block 26, such as filters and wave shapers, e.g. to generatea modulation signal with a desired waveform such as sinusoidal.

In order for the Time-of-Flight principle to remain valid, these TOFsignals can be frequency modulated, but their average frequency must beknown. Further, because each single TOF distance measurement is composedout of multiple integrations derived from multiple acquisition periods,this mean frequency must be the same during these multiple integrations.This becomes an important requirement in case the multiple integrationsare taken consecutively in time. In such case, if the mean frequency isnot kept the same during the multiple measurements, the resultingcalculated distance will be wrong or the calculation therefor will bemade very difficult or inaccurate.

A Timing block 28 is responsible for making sure the average frequencyremains the same during the multiple integrations. In one aspect of thepresent invention this is achieved by making sure that the integrationtime is exactly an integer number of periods of the perturbationfrequency modulating signal. In the event where the perturbationfrequency modulating signal is a symmetrical signal (such as a sine ortriangle wave), the integration time can also be taken as a integermultiple of half the period time of the perturbation frequencymodulating signal. The timing block 28 preferably receives the cleanclock signal from the system clock 22 which is used by the timing block28 to determine that each integration time is the same number of pulsesapplied to the light sources and hence that the average frequency isconstant.

Furthermore the periodic signal applied to the light sources taken overone integration time or measurement duration of a set of integrationtimes or measurement durations has the same spectral content as theperiodic signal taken over any other measurement duration of the set.

Furthermore, background light present in the scene (e.g. sunlight,ambient light) can reduce or destroy the validity of the obtained depthmeasurement, so special care is preferably taken that the exact sameamount of background light is received by the sensor during each of theconsecutive integrations. Therefore, the TOF signals used during theconsecutive integrations must have the same spectral content, and theymust preferably be aligned with the background light.

Both requirements can be fulfilled by the Timing block 28, which usesthe clean clock signal to make sure the average frequency during eachintegration phase remains the same, and to make sure that theintegration phases are perfectly aligned with the ambient light. Forexample, typically 50 Hz or 60 Hz mains frequencies are used, so thatbackground light from lamps will have frequency components relating tothese two common frequencies. 50 Hz background lighting has a differentoptimal timing setting compared to a 60 Hz background lighting.

Application of a Spread Spectrum has a positive impact on the EMIperformance of the overall system, while it has a small negative impacton system noise performance (jitter). Advantageously, the architecturedescribed above can be extended if the mixing block 26 is adapted toallow dynamical increasing or decreasing of the perturbation frequency,effectively increasing or decreasing the Spread Spectrum impact. Such afeature is useful for factory or on-site calibration. As employed inembodiments o the present invention Spread Spectrum impact is minimalwhile still performing within EMI limits.

In accordance with a further embodiment the perturbation modulation canbe set in accordance with the power supplied to the light sources. In alow-power mode, which causes less EM radiation, a lower frequencymodulating algorithm can be used by the mixing block 28, in order toprovide an optimal tradeoff for this mode.

Referring now to FIG. 4, a time-of-flight camera 1, also referred to asTOF camera 1, according to one embodiment of the present invention canbe seen. The time-of-flight camera 1 comprises a an illumination unitwith at least one light source 2, which is in this embodiment of theinvention a LED, for emitting a periodic signal light signal such asmodulated light pulses having a wave length and frequency depending oncharacteristics of the light sources towards a scene. Other lightsources can be used such as OLED's, laser diodes, lasers etc.

The light source 2 is connected to a modulation unit 3, which provides aperturbed modulation signal to the light source for modulation thereof.The modulation unit 3 can be provided as on-chip implementation forachieving reliable control of the perturbed modulation signal.

The time-of-flight camera further comprises a modulation clock or “cleanlock” 4 and a perturbation clock 5, which are both connected to themodulation unit 3. The modulation clock 4 provides a clock signal, ascan be seen in the upper time scale of FIG. 2, as base frequency for thefrequency modulation to the modulation unit 3 and the perturbation clock5 provides a further clock signal as perturbation signal to themodulation unit 3. The perturbation clock 5 provides the perturbationsignal with a perturbation frequency, which is lower than the basefrequency for the frequency modulation.

Other components can be placed between the clocks 4, 5 and themodulation unit 3, such as filters, wave shapers, frequency converters,phase locked loops, etc. Other components can be placed after themodulation unit 3, such as filters, or wave shapers to generate aperiodic signal such as square wave or sinusoidal.

The perturbation applied to the base frequency of the modulation in themodulation unit 3 modifies the base frequency of the modulation withinan interval of +/−5% of the base frequency of the modulation, preferablywithin an interval +/−1.5% of the base frequency of the modulation, evenmore preferably within an interval +/−0.1% of the base frequency of themodulation. As can be seen in FIG. 2, at the points of time marked t₁,t₂, t₃ and t₄, the perturbation changes in respect to its phase from 0°to 90°, 180°, and 270°, respectively.

In this exemplary embodiment of the invention, light source 2,modulation unit 3, modulation clock 4, and perturbation clock 5 areindividual components of the time-of-flight camera 1, but can also beprovided in modified embodiments in functional groups comprising atleast two of the above mentioned components.

The time-of-flight camera 1 further comprises a reception group 6 with areceiver unit 7, an evaluation unit 8 and a processing unit 9. Thereception group 6 is optionally connected to the perturbation clock 5 toreceive the perturbation signal. The reception group 6 is also connectedto the output of the modulation unit 3 as a reference signal. Althoughin this exemplary embodiment of the invention, receiver unit 7,evaluation unit 8 and processing unit 9 are provided together formingthe reception group 6, in modified embodiments of the invention they canbe provided as individual functional units of smaller functional groups.

The receiver unit 7 can comprise several light receiving locations,which are not explicitly shown in the diagram of FIG. 4, and which arealso referred to as pixels. The TOF camera 1 further comprises anoptical system, which is not shown in FIG. 4. By means of the opticalsystem, the periodic light signal emitted from the light source 2 aredirected towards the scene and reflections of objects of the scene aredirected towards the different pixels of the receiver unit 7. Hence,different pixels can receive reflections from different objectsindependently and at the same time.

The evaluation unit 8 evaluates the time-of-flight of the receivedreflections individually for each pixel of the receiver unit 7 underconsideration of the output of the modulation unit and/or theperturbation signal provided from the perturbation clock 5.

The calculation unit 9 derives distance information from thetime-of-flight information provided by the evaluation unit 8 for eachpixel or group of pixels and provides this information via an interface,which is not shown in FIG. 4, to a user or a further processing device.

Now, the process for providing distance information of the scene isdescribed in detail.

The method starts with emitting a periodic light signal such asmodulated light pulses from the light source 2 towards the scene. Theperiodic light signal such as the modulated light pulses is generatedusing the modulation signal provided by the modulation unit 3. FIG. 3shows schematically examples of a frequency signal of the perturbationclock 5 (upper graph) and the modulation signal from the modulation unit3 to the light source 2 (lower graph). The upper part of the diagram ofFIG. 3 indicates the change of the frequency of the periodic lightsignal of the modulation signal from the modulation unit 3 over the timein accordance with the applied perturbation. As can be seen in the lowerpart of the diagram of FIG. 3, the pulses of the modulation signal areprovided to the light source 2 (to form the periodic light signal) witha varying frequency and length accordingly.

Next, reflections of the periodic light signal e.g. the light pulses arereceived from the scene by the receiver unit 7 via the optical system.The reflections are generated by the objects present in the scene.

The steps of emitting the periodic light signal such as modulated lightpulses and receiving the reflections from the scene are repeated and thereceived reflections are integrated by the evaluation unit 8. Asindicated in the lower part of the diagram of FIG. 2, reception ofreflections from the modulated light pulses are started at the points oftime denoted as t₁, t₂, t₃ and t₄. As can be seen in FIG. 2, receivingthe reflections of the periodic light signal e.g. modulated light pulsesis, in this example, always started at a peak of the perturbationfrequency modulating signal, and therefore at the same position of theperturbation frequency modulating signal. The reception is performed inthis embodiment of the present invention for a measurement time over aninteger number such as six full periods of the perturbation modulationsignal and the first of these is marked as “measure”. Accordingly, thestep of receiving reflections of the periodic light signal, e.g.modulated light pulses from the scene is spanned for each performanceover the same number of full perturbation periods and, as can be seen inFIG. 2, a length of the perturbation period is shorter than themeasurement time of each reception of the light pulse. Further, thisoperation provides that in all performances of emitting a periodic lightsignal such as modulated light pulses towards the scene, the averagefrequency of the modulation signal provided to the light source 2 isessentially identical.

In this exemplary embodiment of the present invention, four integrationintervals marked as “measure” are evaluated sequentially. In alternativeembodiments, the received reflections can be integrated all at the sametime in the receiver unit 7.

The evaluation unit 8 evaluates a time-of-flight information for thereceived reflections of the periodic light signal such as the modulatedlight pulses for all pixels under consideration of the perturbationapplied to the base frequency of the periodic light signal, and providesthe time-of-flight information to the calculation unit 9. Thecalculation unit 9 derives distance information for all pixels or groupsof pixels from the provided time-of-flight information and provides thisinformation as distance information of the scene for further processing.The distance information of the scene is therefore provided as anaverage over the four integration intervals. A time, marked as “readout”is used by the reception group 6 for processing the receivedreflections.

A resulting spectrum of an EMC measurement for the time-of-flight camera1 according to this embodiment of the invention has peak energiesprovided more uniformly over at least a part of the frequency range.

FIG. 6 shows another embodiment of a TOF camera or range finding systemaccording to the present invention. The range finding system comprises alight source 49 for emitting periodic light 51 onto a scene 55,preferably focussed onto an area of interest, where the light isreflected. The range finding system further comprises at least one pixel31 for receiving reflected light. In order for the light source 49 toemit modulated light, a signal generator 43 is provided. The signalgenerator 43 generates a first Spread Spectrum perturbed clock signal ormodulation signal on node 48 that is preferably permanently oscillatingat a predetermined average frequency, e.g. at about 10 MHz. This signalgenerator 43 also generates similar second to fifth Spread Spectrumperturbed clock signals (e.g. derived from a single Spread Spectrumclock generator 43) which are delivered onto nodes 44, 45, 46, 47,respectively, having a 0°, 180°, 90° and 270° phase relation with thefirst clock signal on node 48 with respect to the perturbation period. Aperson skilled in the art can also consider using other or more clockphases in the operation scheme, more clock phases leading towards bettermeasurement precision in exchange for a longer measurement time. Theperturbation frequency is within an interval of +/−5% of the basefrequency, preferably within an interval +/−5% or +/−1.5% of the basefrequency, or within an interval +/−0.1% of the base frequency of themodulation. The periodic perturbation can have a sinusoidal ortriangular waveform, for example.

The signal generator 43 can also generate a control signal 41 that isdetermining for a modulation signal alteration means to change themodulation signal, e.g. a control signal 41 that is determining for aselector 58 to select between the second to fifth Spread Spectrumperturbed clock signals, i.e. between the different phases of the clocksignal. Selector 58 is switching sequentially between these four phasesconnecting the input node 42 of a mixer 29 of a detector and mixer stage200 with the second to fifth clock signals on nodes 44, 45, 46 and 47sequentially. At each of these positions selector 58 can stay connectedfor a relaxation period of e.g. about 1 ms.

A further control signal can be generated to determine the position inthe pulse sequence where the start and stop of measurements occurs.Alternatively, the system makes sure that for each integration time theexact same number of periods of the perturbation signal is used.Measurements can be started at the same moment (phase) of theperturbation signal, as the spectral content is not affected as long asthe measurement is over an integer number of half periods.

Buffer 50 drives the light source 49 that emits its light 51 onto thescene 55, preferably focused on the area of interest. Part of this lightwill be reflected, thus generating reflected light 52. This reflectedlight 52 then arrives on an optical focussing system such as a lens 56,through which it is imaged or focussed on a detector 28 inside pixel 31,where the incident fraction is called the reflected modulated light (ML)27.

Indirect light 53 and direct light 54, both originating from secondarylight sources 30 not intended for the TOF measurement, will also bepresent in the scene, impinge on the optical focussing system 56 andthus be focused on the detector 28. The part of this light enteringdetector 28 will be called background light (BL) 26. Light sources 30generating BL include incandescent lamps, TL-lamps, sunlight, daylight,or whatever other light that is present on the scene and does notemanate from the light source 49 for TOF measurement. An aim of thepresent invention is to obtain valid TOF measurements even in thepresence of the signal from BL 26.

ML 27 and BL 26 impinge onto the photodetector 28, and generate,respectively, an ML-current and a BL-current, which are photo-inducedcurrent responses to the impinging BL 26 and ML 27. Detector 28 outputsthese currents to a subsequent mixing means, e.g. mixer 29, for mixingthe current responses to the impinging BL 26 and ML 27 with thephase-shifted clock signal on input node 42. As already stated earlier,this BL 26 can induce a BL-current of up to 6 orders of magnitude higherthan the ML-current induced by the ML 27 received for TOF measurements.

Detector 28 and mixer 29, forming detector and mixer stage 200, can aswell be implemented as one single device, for example as described inEP1513202A1, where the photo-generated charges are mixed generating themixing product current at once.

The detector and mixer stage 200 will generate the mixing products ofthe current responses to the impinging BL 26 and ML 27 withphase-shifted clock signals, and these signals are being integrated onnode 38 by means of an integrator, for example implemented with acapacitor 25, which preferably is kept small, e.g. the parasiticcapacitance of the surrounding transistors. During integration, anautomatic reset of the mixer output signal on the integrator node 38 isperformed.

This may for example be implemented by a comparator 33 triggering areset switch, e.g. reset transistor 32, so that the mixer output signalon node 38 is automatically reset whenever it reaches a reference valueVref, thus avoiding saturation.

In alternative embodiments, not illustrated in the drawings, theautomatic reset of the mixer output signal on the integrator node 38 canbe implemented in several other ways. One of them is triggering a chargepump, instead of the reset switch 32, to add a fixed amount of chargesto capacitor 25 yielding a better noise performance at the cost of somemore complexity.

The mixing products forming the mixer output signal are available in asequential form synchronised with the modulation signal alterationmeans, in the example illustrated selector 58, at integrator node 38. Anoutput driver 24, e.g. a buffer, provides a voltage gain substantiallyof one and current amplification so as to provide a stronger outputsignal at output node 23.

Various modifications are included within the scope of the inventionsuch as application of a discontinuous perturbation modulation to thelight pulses. Further the system may be adapted for dynamicallyincreasing or decreasing the perturbation frequency. Further when thepower of the light pulses is lower, the perturbation frequency can bereduced.

1. A method for providing distance information of a scene with atime-of-flight sensor or camera (1), comprising the steps of emitting aperiodic light signal towards the scene in accordance with a modulationsignal based on a clock timing that has a base frequency spread by aperiodic perturbation with a perturbation frequency and period,receiving reflections of the periodic light signal from the scene,evaluating a time-of-flight information for the received reflections ofthe periodic light signal over a set of a plurality of measurementdurations in accordance with the modulation signal, and derivingdistance information from the time-of-flight information for thereceived reflections, wherein each measurement duration of the set is aninteger or half integer multiple of the perturbation period and over theset of measurement durations the average base frequency is keptconstant. 2-27. (canceled)
 28. Method according to claim 1, wherein themodulation signal taken over one measurement duration of the set has thesame spectral content as the modulation signal taken over any othermeasurement duration of the set.
 29. Method according to claim 1,wherein the step of evaluating a time-of-flight information for thereceived reflections of the periodic light signal comprises integratingthe received reflections of the periodic light signal over each of theplurality of measurements of the set, and combining the results. 30.Method according to claim 1, wherein a discontinuous perturbationmodulation is applied.
 31. Method according to claim 1, furthercomprising dynamically increasing or decreasing the perturbationfrequency.
 32. Method according to claim 1, further comprising loweringthe power of the periodic light signal and reducing the perturbationfrequency.
 33. The method of claim 1, wherein, the periodic light signalis a pulsed or sinusoidal signal.
 34. Method according to claim 1,wherein the perturbation frequency is within an interval of the basefrequency, preferably within an interval +/−5% or +/−1.5% of the basefrequency, or within an interval +/−0.1% of the base frequency of themodulation signal.
 35. A range finding device for providing distanceinformation from a scene, the range finding device being for use with alight source for emitting a periodic light signal towards the scene, therange finding device comprising: a modulation unit (3) for providing amodulation signal for the light source based on a clock timing with abase frequency spread by a periodic perturbation with a perturbationfrequency and period, a reception group (6) with a receiver unit (7), anevaluation unit (8) and a processing unit (9), the reception group (6)being connected to the modulation unit to receive the modulation signal,the evaluation unit (8) being adapted to evaluate time-of-flightinformation from received reflections from the scene over a set of aplurality of measurement durations in accordance with the clock timingthat is spread by the periodic perturbation, the calculation unit (9)being adapted to derive distance information from the time-of-flightinformation provided by the evaluation unit (8) wherein each measurementduration of the set is an integer or half integer multiple of theperturbation period and over the set of measurement durations theaverage base frequency is kept constant.
 36. A range finding deviceaccording to claim 35, further arranged for providing said distanceinformation from a scene according a method for providing distanceinformation of a scene with a time-of-flight sensor or camera (1),wherein said device performs a method comprising: emitting a periodiclight signal towards the scene in accordance with a modulation signalbased on a clock timing that has a base frequency spread by a periodicperturbation with a perturbation frequency and period, receivingreflections of the periodic light signal from the scene, evaluating atime-of-flight information for the received reflections of the periodiclight signal over a set of a plurality of measurement durations inaccordance with the modulation signal, and deriving distance informationfrom the time-of-flight information for the received reflections,wherein each measurement duration of the set is an integer or halfinteger multiple of the perturbation period and over the set ofmeasurement durations the average base frequency is kept constant.
 37. Atiming module for time-of-flight sensor for use with a light source foremitting periodic light signal towards a scene, the sensor being forproviding distance information from a scene, the timing modulecomprising: a modulation unit (3) for providing a modulation signal forthe light source having a clock timing with a base frequency spread by aperiodic perturbation with a perturbation frequency and period, themodulation unit being adapted to provide the modulation signal over aset of a plurality of measurement durations in accordance with the clocktiming that is spread by the periodic perturbation, wherein eachmeasurement duration of the set is an integer or half integer multipleof the perturbation period and over the set of measurement durations theaverage base frequency is kept constant.